How to Use the PV Module String Sizing Calculator
If a series string of PV modules produces a voltage outside what the inverter can handle, it can damage the inverter or create a fire risk. This calculator uses the module's open-circuit voltage (Voc) and operating voltage (Vmp), together with the inverter's maximum input voltage and MPPT tracking range, to find a safe series count range.
The maximum series count is set by keeping the string's total Voc under the inverter's max input voltage, while the minimum series count is set by keeping the string's total Vmp above the inverter's minimum MPPT voltage — below that, energy harvest drops off. The overlap of both conditions is your recommended string size range.
Because module Voc rises in cold weather, NEC 690.7 requires applying a low-temperature correction factor based on the site's record low temperature — which often trims one module off the calculated maximum for a safety margin. Confirm your final string design against the inverter manufacturer's datasheet and a qualified PV designer.
Frequently Asked Questions
Open-circuit voltage (Voc) is the maximum voltage under no load, used to find the maximum series count that stays under the inverter's max input voltage. Operating voltage (Vmp) is the real generating voltage, used to find how many modules keep the string inside the inverter's MPPT tracking range.
Yes. A PV module's open-circuit voltage rises as temperature drops. Per NEC 690.7, applying a low-temperature correction factor for your area's record low can push Voc higher than this calculator's default result, so it's safer to keep one module fewer than the calculated maximum.